1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus.
2. Description of Related Art
In image forming apparatuses according to the related art, such as a copying machine, a printer, and a facsimile, a toner image formed on a photosensitive drum, which is an image bearing member, provided in an image forming portion is transferred onto a sheet, and the sheet is conveyed to a fixing device. Then, the fixing device heats and presses the sheet to fix the toner image to the sheet, and a sheet conveying apparatus discharges the sheet having the toner image fixed thereto to a sheet discharging and stacking portion.
However, in the image forming apparatus according to the related art, for example, when a roller comes into contact with a surface of the sheet having the toner image formed thereon or the rear surface thereof before the toner image that is heated and fixed at a high temperature is cooled down, the surface property of the contact portion is changed or the contact portion dissipates heat from the sheet earlier than the other portions. Consequently, the color of the toner image is changed, or a so-called roller trace, which is viewed as a brilliance difference, occurs, which results in deterioration of image quality. The roller trace is less likely to be visually viewed from a plain sheet, but is more likely to be viewed from a sheet having a high degree of brilliance, such as a projector sheet, glossy paper, or a glossy film.
In order to prevent deterioration of image quality due to the generation of the roller trace, for example, a structure has been proposed in which a roller having an insulating property is used as a conveying roller (discharge roller) of a sheet conveying apparatus contacted with a toner image that is heated and fixed at a high temperature (see Japanese Patent Laid-Open No. 2004-301291).
FIG. 10A is a diagram illustrating the structure of the roller having an insulating property. The roller includes an insulator layer 101 that is formed on a circumferential surface of a metal shaft 100, an uneven portion 102 that is formed on the insulator layer 101, and a surface protecting layer 103 that is formed on a circumferential surface of the insulator layer 101.
Furthermore, in order to prevent deterioration of image quality due to the generation of the roller trace, for example, a structure has been proposed in which a roller having a small contact area with a sheet is used as a roller of a sheet conveying apparatus contacted with a toner image that is heated and fixed at a high temperature (see Japanese Patent Laid-Open No. 2006-145586).
FIG. 10B is a diagram illustrating the structure of the roller. The roller 110 includes a plurality of minute protrusions 110a on an outer circumferential surface thereof that contacts a toner image heated and fixed to a sheet. The minute protrusions 110a enable the roller to come into point contact with the toner image on the sheet before the toner image is cooled down, thereby preventing the generation of a roller trace.
However, in the sheet conveying apparatus according to the related art that uses the roller having an insulating property, the manufacturing cost of the roller (insulating roller) is increased since a material forming the insulator layer is expensive. In addition, the hardness of an insulator forming the insulator layer is low. Therefore, when a skid comes into pressure contact with the roller, the insulator layer is compressed, and an insulating effect is lowered. As a result, a roller trace occurs, and image quality deteriorates. In order to prevent the insulating effect from being lowered, when the urging force of a spring that presses, for example, a skid against the roller, the conveying force (discharge force) of a pair of rollers is lowered. As a result, it is difficult to reliably discharge sheets.
In addition, in the sheet conveying apparatus according to the related art that has a plurality of minute protrusions on the outer circumferential surface thereof, in some cases, a toner parting property deteriorates by the plurality of minute protrusions formed on the outer circumferential surface of the roller. When the toner parting property deteriorates as described above, the following problem arises. When the sheet having the toner image heated and fixed thereto passes through a pair of rollers before the toner image is cooled down, toner on the sheet is peeled off, which results in deterioration of image quality. The peeled toner is likely to adhere to the rollers.
When a contact area between toner and the rollers is small and the heights of contact portions between the minute protrusions and the sheet are not uniform, the minute protrusions formed on the outer circumferential surface of the roller cause high pressure to be locally applied to the sheet. Consequently, toner on the sheet is peeled off, or the sheet is deformed and an uneven portion is formed on the sheet. As a result, image quality deteriorates.
In order to prevent the peeled toner from adhering to the roller, it is considered to form a surface protecting layer on the circumferential surface of the roller. However, when the surface protecting layer is formed, the protrusions on the outer circumferential surface of the roller are gently inclined. As a result, a contact area between toner on the sheet and the roller is increased, and it is difficult to sufficiently prevent the generation of a roller trace.